<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <button id="btn">点我一下</button>
</body>

</html>
<script>
  //获取按钮元素
  var btn = document.getElementById('btn')
  btn.onclick = function () {
    //获取的按钮元素中文本的字号设置为30px
    btn.style.fontSize = '30px';
  };
</script>